Known fuel metering systems control the quantity of fuel to be metered in open-looped fashion, in accordance with operating characteristics such as load, rpm, and temperature. Closed-loop controlled metering systems are also known, but this closed-loop control has not been carried out to the full extent possible; that is, it is not the metered fuel quantity itself which is measured and processed as a feedback signal, but rather only a position signal, relating, for instance, to the position of the control rod. The assumption in such a system is that this positional signal is sufficient to characterize the particular quantity of fuel metered at a particular time. In closed-loop control systems which function mechanically, this assumption is justified, considering the tolerances which exist in a mechanical system. With electronic systems, however, the signal processing is extremely precise and is also substantially independent of the effects of aging, where errors deriving from worn purely mechanical components may have an interfering effect. These mechanical errors occur especially in high-pressure injection systems, such as those used for Diesel engines, and they are caused by effects which are generally described as "aging". Examples of such aging effects are a relaxation of compression springs, wearing down of control edges and the like. All of this can cause imprecise control of the fuel quantity, so that the position of the control rod, for instance, is no longer an exact standard for the quantity of fuel needed.
Such effects of aging are of only lesser importance in terms of the driving behavior per se of a vehicle, because the driver of an appropriately equipped vehicle, as a rule, is interested only in a particular speed of the vehicle, and this is attained by pressing down to a greater or lesser extent on the accelerator pedal.
These errors become problematical when the positional signal of a quantity-determining member and a metered fuel quantity are to be associated; such as, whenever either the initial setting signals for this quantity-determining member or the positional signal picked up from a travel transducer controls further units of the system in an open-loop fashion. As example of this is exhaust recirculation in the partial load range. Thus, it can happen that the positional signal is signalling a partial load state on the part of the internal combustion engine, and feedback of exhaust gas is desired only in this operational state, but the metered fuel quantities may already correspond to full-load operation. The consequences of this are impermissibly high emissions of toxic substances.
A further consideration, when there is an incorrect relationship between the positional signal and the metered fuel quantity, is inconsistency of the rated output of the engine. An illustration of this is the case where more fuel is needed with aging on the part of the injection pump, allowing the engine to attain a higher output than that originally intended, which may have unintended consequences in terms of control.